A 48 year old man, who engaged in regular physical exercise, went to see his physician because of recurrent headaches. Physical examination revealed that the patient had a mean heart rate of 55 beats/min. His physician noted that the patient’s cardiac rhythm varied substantially with the phases of respiration; the heart rate increased during inspiration and decreased during expiration.
The physician diagnosed this patient’s headaches as migraine. She advised the patient to take propranolol, a β-adrenergic receptor antagonist, to relieve the headaches. The physician noted that after the patient had taken the propranolol, the mean heart rate diminished very slightly, and the respiratory fluctuations in heart rate were not appreciably different from those observed before the propranolol was taken.
3. Does the failure of propranolol to induce a substantial change in mean heart rate or in the respiratory fluctuations in heart rate necessarily signify that the patient’s cardiac sympathetic neural activity was negligible at the time he was being examined?
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The function of the heart is to contract and pump oxygenated blood to the body and deoxygenated blood to the lungs. To achieve this goal, a normal human heart must beat regularly and continuously for one’s entire life. Heartbeats originate from the rhythmic pacing discharge from the sinoatrial (SA) node within the heart itself. In the absence of extrinsic neural or hormonal influences, the SA node pacing rate would be about 100 beats per minute. Heart rate and cardiac output, however, must vary in response to the needs of the body’s cells for oxygen and nutrients under varying conditions. In order to respond rapidly to the changing requirements of the body’s tissues, the heart rate and contractility are regulated by the nervous system, hormones, and other factors.
The function of the heart is to contract and pump oxygenated blood to the body and deoxygenated blood to the lungs. To achieve this goal, a normal human heart must contract regularly and continuously, and respond to the changing requirements of the body’s tissues. Here we review how the cardiovascular system is controlled and influenced by not only a unique intrinsic system but is also heavily influenced by the autonomic nervous system as well as the endocrine system.
The autonomic nervous system (ANS) is the component of the peripheral nervous system that controls cardiac muscle contraction, visceral activities, and glandular functions of the body. Specifically, the ANS can regulate heart rate, blood pressure, the rate of respiration, body temperature, sweating, gastrointestinal motility, and secretion, as well as other visceral activities that maintain homeostasis. The ANS functions continuously without conscious effort. The ANS, however, is controlled by centers located in the spinal cord, brain stem, and hypothalamus.
The ANS has two interacting systems: the sympathetic and parasympathetic systems. sympathetic and parasympathetic neurons exert antagonistic effects on the heart. The sympathetic system prepares the body for energy expenditure, emergency or stressful situations, i.e., fight or flight. Conversely, the parasympathetic system is most active under restful conditions. The parasympathetic counteracts the sympathetic system after a stressful event and restores the body to a restful state. The sympathetic nervous system releases norepinephrine (NE) while the parasympathetic nervous system releases acetylcholine (ACh). Sympathetic stimulation increases heart rate and myocardial contractility. During exercise, emotional excitement, or under various pathological conditions (e.g., heart failure), the sympathetic nervous system is activated. The stimulation of the sympathetic nervous system causes pupil dilatation, bronchiole dilatation, blood vessel constriction, sweat secretion, inhibits peristalsis, increases renin secretion by the kidneys, as well as can induce reproductive organ contraction and secretion. In contrast, parasympathetic stimulation decreases heart rate and constricts the pupils. It also increases secretion of the eye glands, increases peristalsis, increases secretion of salivary and pancreatic glands, and constricts bronchioles. Most organs receive innervations from both systems, which usually exert opposing actions. However, this is not always the case. Some systems do not have a response to parasympathetic stimulation. At any given time, the effect of the ANS on the heart is the net balance between the opposing actions of the sympathetic and parasympathetic systems.
Breathing is a function that can be performed both voluntarily and involuntarily. When exercising and recovering from exercise, the depth and rate of breathing are adjusted by the autonomic nervous system. The portions of the autonomic nervous system that controls heart rate, circulation, muscle tension, and many other bodily functions are influenced by conscious breathing. While breathing normally, heart rates usually increase during inhalation and decrease during exhalation. This cyclic change in heart rate, that is driven by breathing, is known as Respiratory Sinus Arrhythmia (RSA).
*Patients with sinus arrhythmia do not experience any cardiovascular symptoms. The sinus node rate can change with inspiration/expiration. The heart rate speeds up with inspiration (since it inhibits your vagal nerve) and decreases with expiration (stimulates vagal nerve).
Sinus arrhythmia is present when the P wave morphology is normal and consistent and the P-P intervals vary by more than 120 milliseconds.
*In the present scenario the doctor is prescribed to treat the migraine headache with propranolol. Propranolol has recently been introduced as a useful agent in the treatment of a variety of cardiac arrhythmias. It slows sinus and atrial tachycardias and can abolish ectopic ventricular rhythms. In addition, propanolol slows the ventricular response in patients with atrial tachycardia, flutter, and fibrillation.
But the main fact is that it is contraindicated in hypotension and bradycardia. Here the doctor fails to determine that the case is sinus arrhythmias during the physical examination and leads to the negative effect of the propanolol
A 48 year old man, who engaged in regular physical exercise, went to see his physician...
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